Synthesis gas containing hydrogen and carbon monoxide is used for a variety of industrial applications, for example, the production of hydrogen, chemicals and synthetic fuels. Conventionally, the synthesis gas is produced in a fired reformer in which natural gas and steam are reformed in nickel catalyst containing reformer tubes at high temperatures (e.g., 850° C. to 1000° C.) and moderate pressures (e.g., 16 to 30 bar) to produce the synthesis gas. The endothermic heating requirements for steam methane reforming reactions occurring within the reformer tubes are provided by burners firing into the furnace that are fueled by part of the natural gas. In order to increase the hydrogen content of the synthesis gas produced by the steam methane reforming (SMR) process, the synthesis gas can be subjected to water-gas shift reactions to react steam with the carbon monoxide in the synthesis gas. Typically hydrogen is recovered by treating the hydrogen rich stream in a H2 PSA. The tail gas from the H2 PSA is utilized as a fuel in the fired reformer to facilitate endothermic reforming reactions.
A well-established alternative to steam methane reforming is the non-catalytic partial oxidation process (POx) whereby a sub-stoichiometric amount of oxygen is allowed to react with the natural gas feed creating steam and carbon dioxide at high temperatures. The high temperature residual methane is reformed through reactions with the high temperature steam and carbon dioxide.
An attractive alternative process for producing synthesis gas is the autothermal reforming (ATR) process which uses oxidation to produce heat and a catalyst to permit reforming to occur at lower temperatures than the POx process. Similar to the POx process, oxygen is required to partially oxidize natural gas in a burner to provide heat, high temperature carbon dioxide and steam to reform the residual methane. Some steam needs to be added to the natural gas to control carbon formation on the catalyst. However, both the ATR and POx processes require separate air separation units (ASU) to produce high-pressure oxygen, which adds complexity as well as capital and operating costs to the overall process.
When the feedstock contains significant amounts of heavy hydrocarbons, SMR and ATR processes are typically preceded by a pre-reforming step. Pre-reforming is a catalyst based process for converting higher hydrocarbons to methane, hydrogen, carbon monoxide and carbon dioxide. The reactions involved in pre-reforming are generally endothermic. Most pre-reformers on natural gas streams operate in the endothermic area and operate adiabatically, and thus the (pre-reformed feedstock leaves at a lower temperature than the feedstock entering the pre-reformer. Another process that will be discussed in this invention is the secondary reforming process, which is essentially an autothermal process that is fed the product from a steam methane reforming process. Thus, the feed to a secondary reforming process is primarily synthesis gas from steam methane reforming. Depending on the end application, some natural gas may bypass the SMR process and be directly introduced into the secondary reforming step. Also, when a SMR process is followed by a secondary reforming process, the SMR may operate at a lower temperature, e.g. 650° C. to 825° C. versus 850° C. to 1000° C.
As can be appreciated, the conventional methods of producing a synthesis gas such as have been discussed above are expensive and require complex installations. To overcome the complexity and expense of such installations it has been proposed to generate the synthesis gas within reactors that utilize an oxygen transport membrane to supply oxygen and thereby generate the heat necessary to support endothermic heating requirements of the steam methane reforming reactions. A typical oxygen transport membrane has a dense layer that, while being impervious to air or other oxygen containing gas, will transport oxygen ions when subjected to an elevated operational temperature and a difference in oxygen partial pressure across the membrane.
Examples of oxygen transport membrane based reforming systems used in the production of synthesis gas can be found in U.S. Pat. Nos. 6,048,472; 6,110,979; 6,114,400; 6,296,686; 7,261,751; 8,262,755; and 8,419,827. There is an operational problem with all of these oxygen transport membrane based systems because such oxygen transport membranes need to operate at high temperatures of around 900° C. to 1100° C. When hydrocarbons such as methane and higher order hydrocarbons are subjected to such high temperatures within the oxygen transport membrane, excessive carbon formation occurs, especially at high pressures and low steam to carbon ratios. The carbon formation problems are particularly severe in the above-identified prior art oxygen transport membrane based systems. A different approach to using an oxygen transport membrane based reforming system in the production of synthesis gas is disclosed in U.S. Pat. No. 8,349,214 which provides an oxygen transport membrane based reforming system that uses hydrogen and carbon monoxide as part of the reactant gas feed to the oxygen transport membrane tubes and minimizes the hydrocarbon content of the feed entering the permeate side of the oxygen transport membrane tubes. Excess heat generated within the oxygen transport membrane tubes is transported mainly by radiation to the reforming tubes made of conventional materials. Use of high hydrogen and carbon monoxide in feed to the oxygen transport membrane tubes addresses many of the highlighted problems with the earlier oxygen transport membrane systems.
Other problems that arise with the prior art oxygen transport membrane based reforming systems are the cost of the oxygen transport membrane modules and the lower than desired durability, reliability and operating availability of such oxygen transport membrane based reforming systems. These problems are the primary reasons that oxygen transport membranes based reforming systems have not been successfully commercialized. Advances in oxygen transport membrane materials have addressed problems associated with oxygen flux, membrane degradation and creep life, but there is much work left to be done to achieve commercially viable oxygen transport membrane based reforming systems from a cost standpoint as well as from an operating reliability and availability standpoint. Also for hydrogen production additional challenges remain. Compared to conventional steam methane reformers, the oxygen transport membrane reforming systems mentioned above produce a synthesis gas containing lower amounts of hydrogen and higher amounts of carbon monoxide (lower hydrogen to carbon monoxide molar ratio); treating this synthesis gas in a hydrogen PSA results in a tail gas having a higher flow and substantial fuel value.
The present invention addresses the aforementioned problems by providing an improved process for hydrogen production using a reactively-driven oxygen transport membrane based system, which comprises of two reactors that can be in the form of sets of catalyst containing tubes—reforming reactor and oxygen transport membrane reactor. Partial oxidation and some reforming occurs at the permeate (i.e. catalyst containing) side of the oxygen transport membranes and a reforming process facilitated by a reforming catalyst occurs in the reforming reactor in close proximity to the oxygen transport membrane reactor. The partial oxidation process, which is exothermic, and the reforming process, which is endothermic, both occur within the oxygen transport membrane based reforming system and thus have a high degree of thermal integration so that heat released in the oxidation process supplies the heat absorbed by the reforming process. Specifically, improvements to the reactively-driven oxygen transport membrane based system include: modifications to the reactively-driven oxygen transport membrane based system to carry out both a primary reforming process in a catalyst filled reforming reactor as well as a secondary reforming process within the catalyst containing oxygen transport membrane reactor, and thermal coupling of the reforming reactor with a source of auxiliary heat to balance the reforming duty between the oxygen transport membrane reactor and the auxiliary heat source. Further improvements to obtain higher hydrogen recovery include subjecting the synthesis gas to water gas shift reactions in a high temperature or medium temperature shift reactor and optionally in a low temperature shift reactor.